As a renewable energy resource, the use of photovoltaic modules is rapidly expanding. A photovoltaic module (also known as a solar cell module) refers to a photovoltaic device for generating electricity directly from light, particularly, from sunlight. With increasing use of photovoltaic modules, comes an increased demand for photovoltaic modules suitable for use in demanding environments. Photovoltaic modules are normally installed in outdoor locations such as on a roof, wall or other supporting structure. Many climate areas where sunlight is plentiful are also places where fire is of concern. Photovoltaic modules installed on building exteriors in fire prone areas may need to be fire resistant.
As shown in FIG. 1, a photovoltaic module 10 comprises a light-transmitting substrate 12 or front sheet, an encapsulant layer 14, an active photovoltaic cell layer 16, another encapsulant layer 18 and a back-sheet 20. The light-transmitting front sheet substrate, also known as the incident layer, is typically glass or a durable light-transmitting polymer film. The encapsulant layers 14 and 18 adhere the photovoltaic cell layer 16 to the front and back sheets and they seal and protect the photovoltaic cells from moisture and air. The encapsulant layers 14 and 18 are typically comprised of a thermoplastic or thermosetting resin such as ethylene-vinyl acetate copolymer (EVA). The photovoltaic cell layer 16 may be any type of solar cell that converts sunlight to electric current such as single crystal silicon solar cells, polycrystalline silicon solar cells, microcrystalline silicon solar cells, amorphous silicon-based solar cells, copper indium (gallium) diselenide solar cells, cadmium telluride solar cells, compound semiconductor solar cells, dye sensitized solar cells, and the like. The back-sheet 20 provides structural support for the module 10, it electrically insulates the module, and it helps to protect the module wiring and other components against the elements, including heat, water vapor, oxygen and UV radiation. The back-sheet needs to remain intact and adhered to the encapsulant for the service life of the photovoltaic module, which may extend for multiple decades.
Multilayer laminates have been employed as photovoltaic module back-sheets. One or more of the laminate layers in such back-sheets conventionally comprise a highly durable and long lasting polyvinyl fluoride (PVF) film which is available from E. I. du Pont de Nemours and Company as Tedlar® film. PVF films resist degradation by sunlight, they provide a good moisture barrier, and they are less prone to burning or melting than films made of many other polymers. PVF films are typically laminated to other less costly polymer films that contribute mechanical and dielectric strength to the back-sheet, such as polyester films, as for example polyethylene terephthalate (PET) films. Other conventional back-sheet laminates are comprised wholly of polyester films, but such back-sheets have been found to experience delamination and they are less resistant to heat and fire than PVF-based films. Conventional back-sheet polymer films, including PVF films and PET films, will burn or melt when exposed to an open flame. Fire resistant back-sheet laminates have been made that incorporate metal foils (US Patent Application Publication No. 2008-0053512) or metal plates (Japan Patent Application Publication No. 2001-036-116). However metal foils and plates are difficult to permanently adhere to other polymer back-sheet layers and they can hinder the electrical insulation properties of the back-sheet.
There is a need for a back-sheet for photovoltaic module that does not readily burn or melt when exposed to fire. There is also a need for such a back-sheet laminate that resists delamination over long periods of time. There is a further need for a back-sheet that offers excellent moisture resistance, durability, and heat and fire resistance.